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Why do overheated brake drums contribute to fade in drum brakes?
- A.The drum shrinks tightly onto the shoes when hot
- B.The hot drum expands outward, away from the linings, so contact and effective pressure are reduced
- C.Heat makes the drum surface magnetic
- D.Expansion improves the pushrod angle
Correct answer
B. The hot drum expands outward, away from the linings, so contact and effective pressure are reduced
Why
As a drum grows with heat, the shoes must travel farther and may contact less of the surface at full pressure. Combined with lower friction from hot linings, that expansion is a major part of drum brake fade.
Manual topic: Air Brakes - Brake Fade and Heat · From the Air Brakes practice bank (AB-177).